This invention relates to a method of producing a biaxially stretched polyester film wherein the bowing phenomenon is improved by controlling conditions of lateral stretching. The polyester film is useful for a support for a photographic film.
Supports for a photographic film are needed to have the bowing phenomenon as small as possible. It is considered that the bowing phenomen is due to the distribution of molecular orientation in the width direction occurring caused by the thermal shrinkage at the central portion which is not fixed by a clip during the process of stretching a longitudinally stretched film laterally by a tenter.
The bowing is defined and evaluated by drawing a straight line in the width direction of a longitudinally stretched film at the entrance of a tenter, and measuring the bowed degree, i.e. the distance between the top of the bowed line and the straight line connected both sides of the film (B in FIG. 3) per the width of the film (W in FIG. 3) at the exit of the tenter. The less the bowing is, the less the distribution of properties in the width direction is, and accordingly, is preferable for the support for a photographic film.
Some trials were made for decreasing the bowing, and for example, the method disclosed in Japanese Patent KOKAI 3-193328 was made for the purpose of improving the bowing which comprises providng an intermediate zone having a length of the film width or more between the stretching zone and the heat setting zone, and cooling the film to the glass transition temperature or less. However, according to this method, the improvement in the bowing was not exhibited in the case of polyethylene-2,6- naphthalate film.
Japanese Patent KOKAI 3-216326 and 4-142916-8 disclose some modifications of the above method, such as restretching in the cooling process, relaxing in the lateral direction in the cooling process or the like. However, these methods were also not effective for the improvement in the bowing as to polyethylene-2,6- naphthalate film.
Moreover, the above methods are difficient in the care for raising a film forming speed, such as cooling conditions, and are inferior in generality.
Japanese Patent KOKAI 3-193329 and 3-207632 disclose the modification of the above method which provides nip rolls in the intermediate zone. However, nip rolls tend to generate abrasion and lateral unevenness, and therefore, the method is not suitable for the production of a support for a photographic film.
An object of the invention is to provide a method of producing a biaxially stretched polyester film having a small bowing, an excellent resistance to cleavage, and adaptability to the variation of film forming speed, and suitable for a support for a photographic film.
The present invention provides a method of producing a biaxially stretched polyester film by stretching a cast film sequentially first in the longitudinal direction and then in the lateral direction which has achieved the above object, which comprises stretching the longitudinally stretched cast film laterally at a temperature higher than the glass transition temperature of the polyester film from by 5xc2x0 C. to by 50xc2x0 C., staying the laterally stretched film in an intermediate zone for a period from 3 to 30 seconds which has been controlled at a temperature from higher than the lateral stretching temperature by 20xc2x0 C. or more to lower than a heat setting temperature by 20xc2x0 C. or less, and then heat-setting at the heat setting temperature which is lower than the melting point of the polyester film from by 30xc2x0 C. to by 5xc2x0 C.